1. Field of the Invention
The present invention relates generally to ultrasonic bonding. More particularly, the present invention relates to a method for bonding two or more conductive members using ultrasonic energy.
2. Disclosure Information
It is well known that ultrasonic energy may be used to join various materials such as plastics and metals. Typically the members to be joined are sandwiched together, placed between a tuned ultrasonic horn and an anvil, clamped together tightly by the horn and anvil with a given amount of force, excited with ultrasonic energy by oscillating the horn which tends to either melt together or intermesh the sandwiched members at least at the interface therebetween, and (if necessary) held until the melted or intermeshed members cool. Plastic members are joined together by reciprocally exciting the members in a direction generally orthogonal to the plane defined between the interfacing surfaces of the sandwiched members. (Hereinafter, this method of ultrasonic excitation shall be referred to as "reciprocal" or "orthogonal".) This causes the plastic material at the interface between the members to melt and coalesce together. Metal members, on the other hand, are joined together by exciting one of them in a direction generally tangent to their interfacing surfaces (i.e., generally parallel to the plane defined between the interfacing surfaces) while rigidly holding the other member, causing the interfacing surfaces of the metal members to shear against each other. (Hereinafter, this method shall be referred to as "shearing".) This method utilizes a type of cold-phase friction welding in which no significant melting of the metals occurs. Rather, the shearing vibration produces a scrubbing action between the interfacing metal members that disrupts the oxides at the interface therebetween, causing metal particles at the surfaces to intermingle and diffuse across their respective surface boundaries, thereby creating a strong metallurgical bond between the members.
Ultrasonic methods are often utilized in the electronics industry for various purposes, such as when bonding gold connecting wires between a chip's substrate mounting pads and the associated leads. Another electronics application which could benefit from the application of ultrasonics is the connecting of flex circuits to terminal strips or to other flex circuits. FIG. 1 shows a typical prior art application in which a flex circuit 10 is used to electrically connect two printed circuit boards (PCBs) 12. The flex circuit shown here comprises a number of metallic conductors 14 carried on or at least partially within a flexible plastic carrier or substrate 16. Each conductor is electrically and mechanically connected to a metallic terminal pad 18 at either end. Typically such connections are made by soldering, but this can be damaging to the flex circuit and/or the PCBs because of the high temperatures involved in this process, particularly if either the flex circuit substrate or PCB substrate is constructed of a low melting point plastic material.
If the flex circuit 10 has metallic wire conductors 14, then the flex circuit can be trimmed or stripped such that bare wire portions 14b are exposed, as shown in FIG. 1. Then, shearing ultrasonic motion could be used to connect the bare wires 14b to their respective metallic terminal pads 18. However, trimming the wires in this way is tedious and time-consuming, and it is often difficult to align the wires with their respective terminals since the wires are prone to bending when handled. Yet, the alternative--not trimming the wires--presents even more problems. It is difficult to use ultrasonics to bond the conductors in untrimmed flex circuits to other metallic conductors because if reciprocal ultrasonic motion is used the plastic will melt but the metal conductors may not weld or fuse together due to the absence of shearing motion, but if shearing motion is used the plastic will not melt and the metal surfaces (e.g., the wires and the terminal pads) may not achieve the intimate face-to-face contact necessary to ultrasonically bond metals. The problem is even further exacerbated if the plastic substrate is very flimsy (as is the usual case) because any shearing motion would tend to tear the substrate, or if the conductors are not wires but instead depositions of conductive ink. In addition to these problems is the drawback that once any satisfactory metal-to-metal connection might somehow be made, the joint would not be easily undoable and redoable--although this would be desirable if any repair or rework had to be done to the flex circuit, or to the PCBs or other members to which it is connected.
It would be desirable, therefore, to provide a method of connecting flex circuits and other conductive members to PCBs, to other circuit members, or to each other, without the need to trim and expose the metallic conductors. It would further be desirable if such connections could be undoable and redoable.